The present invention relates to a heating apparatus using induction heating. Specifically, the present invention concerns a fixing apparatus which is used for an electrophotographic copying apparatus, printer, etc. using toner as a visualizing material and fixing a toner image.
A fixing apparatus installed in a copying apparatus using electrophotographic processes heats and melts the developer, i.e., toner formed on a fixing member to fix the toner on the fixing member. There are widely known methods of heating toner available for the fixing apparatus such as using radiant heat from a filament lamp, using a flash lamp as a heat source, etc.
The fixing apparatus using a filament lamp uses light and infrared rays from the filament lamp to heat a roller around the lamp by radiation. The thermal conversion efficiency is 60% to 70% in consideration of the loss of heat converted from light, the efficiency of transmitting heat to the roller by heating air in the roller, etc. It is known that a long warm-up time is required.
Jpn. Pat. Appln. KOKAI Publication Nos. 9-258586, 8-76620, and the like propose a fixing apparatus using an induction heating apparatus as the heat source.
Jpn. Pat. Appln. KOKAI Publication No. 9-258586 discloses a fixing apparatus which applies an electric current to an induction coil formed around a core along a rotating shaft of a metal roller and generates an induction current in the roller to generate heat from the roller.
Jpn. Pat. Appln. KOKAI Publication No. 8-76620 discloses the fixing apparatus which comprises an induction film including a magnetic field generation means and a pressure roller adhered to the induction film. This fixing apparatus transports a recording medium between the induction film and the pressure roller and heats the induction film to fix toner on the recording medium.
The fixing apparatus used for copying apparatuses is subject to a specific problem of unevenly generating temperature on the metal roller or the film due to an ununiformed size of paper to be fixed (paper passage width). It is requested to shorten the time to warm up the fixing apparatus.
In order to prevent uneven temperature for the paper passage width, there are provided a plurality of induction coils in accordance with the paper passage width along an axial direction of a fixing roller to control electric power supplied to each coil. This example is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-206813. The fixing apparatus disclosed in this publication uses a plurality of detection points to detect heating of the fixing roller and controls the electric power supplied to the respective coils based on the temperature detected at each detection point.
Jpn. Pat. Appln. KOKAI Publication No. 2001-185338 discloses an example of providing a plurality of induction coils for an image forming apparatus using an induction heating apparatus in order to eliminate uneven heating. When a plurality of coils is powered, the example changes the high-frequency power supplied to any coil to the parallel connection. When a plurality of coils is powered simultaneously according to the example in this publication, each coil is connected to a common (same) high-frequency power supply, providing the same phase to electric current supplied to respective coils. It is possible to independently set the power supplied to each coil.
Jpn. Pat. Appln. KOKAI Publication No. 2-270293 discloses an induction heating apparatus having two induction coils. There is provided a zero-voltage detection circuit to detect a zero point of the alternating-current (input) power supply. The publication discloses changeover of electric current supplied to a targeted coil by passing the zero point (0 volt) of the alternating-current (input) power supply. During the changeover of electric current supplied to a targeted coil, an impulse sound (interference sound) occurs between the coil and the roller. To prevent this sound, there is disclosed provision of a specified time interval at the changeover time.
According to the method of driving coils disclosed in the above-mentioned Jpn. Pat. Appln. KOKAI Publication No. 2000-206813, the power supplied to a plurality of coils changes simultaneously. Because of this, a frequency difference occurs between high-frequency currents supplied to respective coils, causing an interference sound (buzzing). Further, there must be independently provided an apparatus to detect the magnitude of power supplied to each coil. In addition, the warm-up time is prolonged when every possible effort is made to uniform the axial temperature of the metal roller.
Jpn. Pat. Appln. KOKAI Publication No. 2001-185338 discloses the common high-frequency power supply apparatus to which respective coils are connected in order to prevent an inverter's interference. However, there are not disclosed actual control timings, control methods, etc. in detail. Nothing is disclosed about a method of shortening the warm-up time.
The method of driving coils disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-270293 uses the soft start feature to prevent occurrence of an excess rush current or to prevent application of a power larger than the controlled one. Soft start is a method of driving coils for preventing an excess rush current from occurring. When a coil is powered, the method feeds back the power by gradually applying an output smaller than the specified output value until this specified value is reached.
When the soft start is performed each time each coil is powered, the heating efficiency degrades and the warm-up time increases. There also arises a problem of increasing the amount of the varying power supplied to each coil. The power supplied to each coil is switched by passing the zero point of the alternating-current power (input) voltage and providing a specified time interval. This causes a flicker etc. caused by the power fluctuation when the coils are changed.